Relying on SO2 for cooling kills thousands, wrecks biodiversity, and fails.

But a third relationship between non-carbon pollution and the temperature of the planet is far more horrifying. This is not the problem of plastic but of “aerosol pollution”—the blanket term for any particles suspended in our atmosphere. Aerosol particles actually suppress global temperature, mostly by reflecting sunlight back into outer space. In other words, all of the non-carbon pollution we’ve exhausted from our power plants and our factories and our automobiles—suffocating some of the largest and most prosperous cities of the world and consigning many millions of the lucky to hospital beds, and many millions of others to early deaths—all of that pollution has been, perversely, reducing the amount of global warming we are currently experiencing. How much? Probably about half a degree—and possibly more. Already, aerosols have been reflecting so much sunlight away from the earth that, in the industrial era, the planet has only heated up two-thirds as much as it would have otherwise. If we had somehow managed to produce precisely the same volume of carbon emissions since the beginning of the Industrial Revolution as we have, while somehow keeping the skies clear of aerosol pollution, the temperature rise would be half again higher than it is now. The result is what the Nobel laureate Paul Crutzen has called a “Catch-22” and what the climate writer Eric Holthaus has described, perhaps more incisively, as a “devil’s bargain”: a choice between publichealth- destroying pollution on the one hand, and, on the other, clear skies whose very clearness and healthiness will dramatically accelerate climate change. Eliminate that pollution and you save millions of lives each year, but also create a dramatic spike in warming. That would bring us to between 1.5 and 2 degrees warmer than the preindustrial baseline— pushing us right up to the threshold of 2 degrees of warming, long thought to be the border separating a livable future from climate catastrophe. For almost a generation now, engineers and futurists have contemplated the practical implications of this phenomenon, and the prospect of suppressing global temperature with a program of suspended particles—that is, polluting the air on purpose to keep the planet cooler. Often grouped together under the umbrella term “geoengineering,” this prospect has been received by the public as a worst-case scenario, nearly science fiction—and has, in fact, informed much of the recent sci-fi that has addressed itself to the climate crisis. And yet it has gained a terrific amount of currency among the most concerned climate scientists, many of whom will also note that none of the quite modest goals of the Paris climate accords can be achieved without negative-emissions technologies —at present prohibitively expensive. Carbon capture may indeed prove to be “magical thinking,” but the cruder technologies—we know these will work. Rather than sucking carbon out of the atmosphere, we could shoot pollution into the sky on purpose; perhaps the most plausible version involves sulfur dioxide. That would turn our sunsets very red, would bleach the sky, would make more acid rain. It would also cause tens of thousands of additional premature deaths each year, through its effect on air quality. A 2018 paper suggested it would rapidly dry the Amazon, producing many more wildfires. The negative effect on plant growth would entirely cancel out the positive effect on global temperature, according to another 2018 paper; in other words, at least in terms of agricultural yield, solar geoengineering would offer no net benefit at all. Once we began such a program, we could never stop. Even a brief interruption, a temporary dispersal of our red sulfur umbrella, could send the planet plunging several degrees of warming forward into a climate abyss. Which would make whatever installations were sustaining that umbrella quite vulnerable to political gamesmanship and terrorism, as its advocates themselves would acknowledge. And yet many scientists still describe geoengineering as an inevitability—it’s just so cheap, they say. Even an environmentalist billionaire, going rogue, could make it happen on their own.